1. Field of the Invention
The invention relates to the field of magnetic resonance imaging methodologies in which dual imaging modes are employed and to electric motors used in the MRI environment.
2. Description of the Prior Art
High magnetic fields used in magnetic resonance imaging (MRI) do not allow the employment of conventional motors due to various incompatibility issues. Magnetic resonance imaging (MRI) is a well-accepted tomographic technique that can be used to acquire high-resolution anatomical three dimensional images of the human body. Although MRI could also provide functional or metabolic information its sensitivity is low compared to other medical imaging techniques such as positron emission tomography (PET) or single photon emission tomography (SPECT). If one wishes to combine the high resolution advantages of MRI with the high sensitivity of nuclear imaging techniques such as SPECT or PET one normally would have to perform image acquisitions separately and then co-register the resultant images either by software techniques or using fiducial markers. Although such co-registration approaches work on rigid organs such as the head it is impossible to perform them accurately on other less rigid body parts such as the abdomen.
Even if the spatial co-registration could be achieved, temporal coregistration cannot be done in the time domain for combined dynamic studies using both imaging techniques simultaneously. This has forced researchers to explore the development of multimodality devices that can integrate a high sensitivity metabolic imaging device such as PET or SPECT into a magnet for fully co-registered PET or SPECT-MR imaging. Some of the challenges one faces in such endeavors include magnetic field compatibility of detectors, data acquisition electronics, and collimators (if there are any). Another difficulty that arises when the detector has to undergo rotational scanning for collecting different angular views is the availability of a motor that can operate close to the magnet bore without affecting the MR image quality or endangering the patient.
A magnetic resonance imaging (MR) compatible motor has been described in “A New Type of Motor: Pneumatic Step Motor” was published by Stoianovici et al in the IEEE/ASME Trans on Mechanotronics v.12 pp 98-106. The motor produces a rotary motion by discrete displacement by sequentially pressurizing the three ports of the motor. Yamamoto developed and evaluated an MR compatible electrostatic motor that is based on thin plastic films and flexible printed circuit technology. They have shown that the linear motor did not affect MR images when operated at distances over 60 cm from the magnet. Although the motor is MR compatible the amount of torque that can be generated limits its use to low torque applications that are more along the line of medical robotics.
A second example is described in Yamamoto et al. “Evaluation of MR-compatibility of Electrostatic Linear Motor” published in Proc. 2005 IEEE Int. Conf. on Robotics & Automation, Barcelona, Spain. This is a high-power electrostatic motor made of paramagnetic materials. Its operation is based on the induced Lorentz forces moving one segment of the motor against the other in a linear movement. Stoianovici's pneumatic step motor that is also compatible with magnetic fields that are used in MR systems. This step motor provides rotational motion and has been shown to deliver torques up to 0.6 N-m. The specific aim of both of these developments is to offer a motor that could be used in MR-guided robotic surgery.
The problem of providing a practical motor for use in an MRI environment is not completely solved for applications that require a large torque to rotate heavy components such as nuclear detectors with collimators. None of the solutions described above satisfactorily resolve this need.
Through the use of highly specific radiolabeled molecular probes, nuclear imaging techniques such as single-photon emission computed tomography (SPECT) can provide insight into a wide range of biological processes with demonstrated applications in neurology, cardiology, oncology and, more recently, stem cell research. However, the relatively poor spatial resolution of radionuclide techniques can make unambiguous localization of the probes extremely difficult, especially when the images lack significant anatomical detail for reference. Limited spatial resolution can also hamper quantification of the probe concentration, especially when localized in small volumes. A unique advantage of SPECT over position emission tomography (PET) is that simultaneous multiple isotope imaging is also possible if the detector has high energy resolution. This opens up the possibility of labeling different molecules with different radioisotopes and performing multidimensional molecular imaging to investigate various biological processes simultaneously.
In contrast to SPECT, magnetic resonance imaging (MRI) can provide exceptionally high spatial resolution anatomical information as well as localized chemical and physical information (i.e., metabolite concentrations, water diffusion characteristics). SPECT and MRI each have their respective advantages and limitations. Integrating these two modalities in a synergistic manner would allow researchers to exploit the strengths of both techniques. For example, MRI data can be utilized to improve the accuracy and spatial resolution of the reconstructed SPECT images. Such improved resolution should reduce partial volume effects, where inaccuracies in quantification of the radiotracer concentrations can occur in smaller structures. This effect becomes significant, for example, in the interpretation of SPECT images of tumors, where a measured decrease in the uptake of a radiotracer following treatment could indicate tumor shrinkage, a change in biological function, or both. Segmentation of the MR images can be used to facilitate attenuation correction of the nuclear projection data, also improving the accuracy of the SPECT reconstruction. Anatomical MR images can provide a reference for the SPECT images, allowing for improved localization. The ability to acquire SPECT and MRI data simultaneously would open up new research opportunities in dynamic imaging using both SPECT radionuclides and MRI contrast agents at the same time with optimum spatial and temporal co-registration. This would provide motivation for the development of appropriate bi-functional imaging probes. Additional advantages of simultaneous SPECT and MRI measurements over separate or sequential acquisitions include the reduction of co-registration errors, decrease in the overall scan time, and the possibility of using the MR images to correct for motion artifacts in the SPECT data. A comprehensive rationale for combining SPECT with MRI was given by Wagenaar et al.
While the integration of SPECT and MRI offers numerous advantages and new opportunities, it also presents many technological challenges. The SPECT detectors must function within an operating MRI scanner. Likewise, the SPECT hardware must not significantly perturb the MR images. Due to these challenges, the development of a combined SPECT and MRI system (henceforth called MRSPECT) is in its infancy, and a very limited amount of research has been reported to date. Breton et al., and Goetz et al., used a strategy similar to PET-CT systems in which a small animal SPECT system was brought in close proximity to a separate MRI system. While they demonstrated excellent results, they utilized a substantially low magnetic field (0.1 T) and performed sequential SPECT and MR imaging. Meng et al., presented the design of an MR-compatible SPECT system for mouse brain imaging based on cadmium-zinc-telluride (CZT) nuclear radiation detectors. While they investigated the effects of the SPECT and MRI components on each other, they confined their disclosure to the use of a 57Co point source for SPECT imaging and were not able to acquire simultaneous SPECT and MRI experimental data. We recently reported on the design and operation of an MRSPECT system also based on CZT detector technology. In that disclosure, the effects of the SPECT and MRI components on each other were characterized through various phantom experiments. The results demonstrated the feasibility of co-registered, simultaneous SPECT and MR imaging.